Sodium bisulfate is a sodium salt with the chemical formula NaHSO4. Chemically, it is an acid salt. As opposed to neutral salts formed in acid-base neutralization reactions, acid salts are salts formed by partial neutralization of acids, which leaves one or more hydrogen atoms intact. As can be guessed from the name and the formula, it is formed by the partial neutralization of sulfuric acid (H2SO4) by a sodium compound such as sodium hydroxide (NaOH) or sodium chloride (NaCl).
Did You Know?Sodium bisulfate is also known as sodium acid sulfate and bisulfate of soda. Its IUPAC name is sodium hydrogen sulfate.
It can be obtained by the following methods:
NaOH + H2SO4 ➙ NaHSO4 + H2O
If 2 molecules of sodium hydroxide were present for every molecule of sulfuric acid, this reaction would form sodium sulfate (Na2SO4) instead of sodium bisulfate. Thus, it is the low amounts of sodium hydroxide that make this reaction possible and make the product acidic instead of neutral. This reaction forms the monohydrate form of sodium bisulfate.
Another method of procuring sodium bisulfate is as follows:
NaCl + H2SO4 ➙ NaHSO4 + HCl
This method requires the sodium chloride-sulfuric acid mixture to be heated, since acids don't normally react with neutral salts at room temperature. This reaction forms the anhydrous form of sodium bisulfate. The gaseous hydrogen chloride obtained as a byproduct in this reaction is then dissolved in water to form hydrochloric acid, a widely used chemical.
Sodium bisulfate consists of a negatively charged hydrogen sulfate, or bisulfate, ion (HSO4-) attached to a positively charged sodium ion (Na+). The bisulfate ion consists of one sulfur atom, showing an oxidation state of +6, joined by double bonds to two oxygen atoms, by a single bond to a third oxygen atom, and by a single bond to a hydroxyl ion. The oxygen atom with the single bond, which leaves it with an oxidation state of -1, bonds with an Na+ ion.
For those of you who may find it slightly difficult to understand the 3D structure, here's a simpler model.
Sodium bisulfate is a white powder. The monohydrate form is granular, whereas the anhydrous form is a hygroscopic amorphous powder. However, it is not violently hygroscopic like phosphorus compounds, and the property doesn't become a significant hindrance in its storage and transport.
The melting point of the monohydrate form is 137.3°F (58.5°C). The anhydrous form, however, melts at a much higher 600°F (315°C). The latter is also the point where it decomposes into sodium pyrosulfate (Na2S2O7) and water.
The density of sodium bisulfate also varies according to its crystallization state. The monohydrate form has a density of 1.8 g/cm3, but the anhydrous form has a density of 2.7 g/cm3.
It has a very high solubility in water: 50 g/100 mL at 0°C. For comparison, table salt has a solubility of about 36 g/100 mL of water.
Due to its acidic nature, many uses of sodium bisulfate are based on its ability to lower the pH of a solution.
► It is used in metal finishing and jewelry making to remove layers of oxidized metal on the surface.
► It is used in various household cleaning products.
► It is used to prepare water (on a large-scale and not potable, such as swimming tanks) for chlorination. The addition of sodium bisulfate makes chlorination more effective.
► It is used as a food additive in various food items; for instance, as a leavening agent in bakeries and as a part of anti-browning products applied to fresh-cut fruits. It is also used in minor amounts in various sauces, dressings, etc.
It is not harmful to humans in small amounts, but inhaling or ingesting large quantities, at once or chronically, can cause severe skin and eye irritation, as well as nausea and digestive tract damage.